1. Field of the Invention
The present invention relates to an image forming apparatus for effectively performing an image processing on image data in which a vector object and a raster object coexist.
2. Description of the Related Art
Up to now, when outputting an achromatic-color (gray) image using a color printer, it is typically possible to express an achromatic color by mixing several colors using a predetermined color conversion table.
According to the principle of subtractive color mixing, the achromatic color can be expressed by performing printing with the three primary colors, yellow (Y), magenta (M), and cyan (C), being mixed in equal proportions.
However, the thus-expressed achromatic color is not as sharp as an achromatic color produced using a single color of black (K), and can produce an unusual resultant product such as a color drift.
To solve the above-mentioned problem, an image forming apparatus is provided with a line-drawing extraction circuit wherein when dots that compose a black character or a black line drawing are extracted from composited image data obtained by compositing color image data and binary black and white image data, bits corresponding to the extracted dots within a bitmap memory are set to “1”. The image forming apparatus is configured so that when a bit corresponding to a dot of a color signal within the bitmap memory is “1”, the color signal is output with CMY and K converted to 0 and a maximum value, respectively, and printing thereof is performed with only K ink.
Accordingly, the image forming apparatus is configured so that when a part of the data related to black is detected from the subject image data, that part is expressed in a single color of black (K), thereby attempting to prevent color drift or the like from occurring in the outputted image. When the printing of an achromatic color including not only black, but also gray is similarly performed using only the K ink, the same result can be obtained.
However, the image forming apparatus described above does not distinguish between a raster object (for example, a scan image) and a vector object (for example, a character) in the subject image data.
For example, in the image forming apparatus, when the image data includes a vector object, the vector object is not composed of dots. Therefore, even when the vector object is drawn in the achromatic color, the image forming apparatus still expresses the vector object in multicolor (for example, CMY or CMYK).
Accordingly, in the image forming apparatus, a color processing is still required even to reproduce the vector object in the achromatic color.
Therefore, in a situation such as illustrated in FIG. 10, where two objects whose areas overlap each other are both black and where one of the objects is composed of a vector object and the other a raster object, output formats are a CMYK output and a K output, respectively, and hence there is a blur in the overlapping area, resulting in reduced visibility.
Another image forming apparatus expresses a raster object and a vector object in CMY or CMYK. In this situation, mutually different color conversion tables are generally used to perform color conversion. This is due to the fact that the raster object is image data for reproducing colors as closely as possible to that which a human sees, while the vector object is designed to express characters and lines more sharply.
In FIG. 18 an example of a raster color table and a vector color table are illustrated that are included in a conventional image forming apparatus. As illustrated in FIG. 18, even when an RGB input value of “(50, 50, 50)” for the raster object and an RGB input value of “(30, 30, 30)” for the vector object are different from each other, both produce the same CMYK output value of “(100, 100, 100, 100)”. As a result, the vector object and the raster object assimilate each other, thereby making the output image not as sharp.
Specifically, in the situation illustrated in FIG. 19, where image data is obtained by using a watermark (electronic watermark) that reads “ABCDEFGHIJ” and a scan image with an overlap, it is possible to clearly distinguish an overlapping portion that corresponds to the overlap on a monitor (see the left half of FIG. 19). However, in the actual outputted image a part of the watermark is assimilated into the scan image and has reduced visibility (see the right half of FIG. 19).